Silicon carbide (SiC) is a semiconductor having excellent physical properties such as an approximately three times larger band gap, an approximately two times larger saturated drift velocity, and an approximately ten times larger dielectric breakdown field strength compared with silicon (Si). Consequently, silicon carbide has been developed as a material of a power semiconductor device, and a Schottky barrier diode (SBD) manufactured with SiC is now commercially available.
The SBD is composed of a SiC single-crystal substrate which is a slice in a wafer shape from a SiC bulk single-crystal grown by a sublimation method or the like, a SiC epitaxial film grown from a surface of the SiC single-crystal substrate by a chemical vapor deposition (CVD), a Schottky electrode deposited on a surface of the SiC epitaxial film by methods such as sputtering and vacuum evaporation, and an ohmic electrode deposited on the rear face of the SiC single-crystal substrate.
One of the challenges for a Schottky barrier power semiconductor device such as an SBD is to reduce power loss during operation, and technical developments therefor have been carried out. A power loss of an SBD based on the total amount of power loss during current passage in a forward direction and power loss due to a leak current during operation at a reverse-direction voltage depends on Schottky barrier height (SBH) in a bonded interface between the Schottky electrode and the SiC epitaxial film.
For instance, the power loss density of an SBD at 50% duty cycle can be described as ½ (VfJf+VrJr) (Non Patent Document 1). Here, Vr is a reverse-direction voltage, Jf is a forward-direction current, Vf is a forward-direction voltage, and Jr is a reverse-direction current. The evaluation of an SBD is represented by Vr and Jf. On the other hand, Vf and Jr depend on the SBH. As an example, in the case the power loss of 4H-SiC SBD is calculated where Jf is 100 Acm−2 and Vr is 4 kV, the power loss is minimized when the SBH is in the range of 1.18 to 1.3 eV at 25 to 200° C.
An SBD having a reverse-direction withstand voltage in the range of about 0.6 to 5.0 kV is used frequently. For such a reverse-direction withstand voltage, the power loss is minimized when the SBH is in the range of about 0.9 to 1.3 eV. However, in the case a Schottky electrode is formed of Ni or Ti, which has been used as a material of a Schottky electrode, the SBH thereof is approximately 1.6 eV for Ni and 0.8 eV for Ti, and the power loss of the SBD cannot be minimized.
As a technique to control the SBH of an SBD provided with a Schottky electrode made of Ni or Ti, it is known that these metals are deposited on a SiC epitaxial film and are then subjected to thermal treatment. However, the thermal treatment increases a value of an ideal factor (n factor) that is a parameter indicating a performance of an SBD, and makes the value greatly deviate from the ideal value of 1.
In general, in the case a current passing through the Schottky barrier interface passes only over the barrier, that is, in the case only thermal diffusion current transport takes place, a current is increased exponentially against a voltage, and the current value is represented by exp(eV/kT)−1 (where e is an elementary electric charge, V is a voltage, k is a Boltzmann's constant, and T is a temperature). However, in the case a current passes not only over the barrier but also through the barrier by tunneling or the like, a current flows even if a voltage is low, and the current value is deviated from the above formula. Consequently, the voltage V is replaced by V/n corresponding to the formula apparently, and the current value is represented by exp(eV/nkT)−1, where n is the ideal factor. In the ideal case where only thermal diffusion transport current flows, n is 1. However, in the actual case a current other than the thermal diffusion transport current flows due to many kinds of causes, a value of the n factor is larger than 1.
As described above, in the case a thermal treatment is carried out to control the SBH after Ni or Ti is deposited, a value of the n factor becomes greatly larger than 1, which deteriorates the performance of an SBD and increases a leak current during an operation at a reverse-direction voltage for instance.
For a silicon carbide semiconductor device, a most typical crystal face that forms a Schottky electrode is a (0001) Si face inclined by several degrees in a direction of [11-20] or [01-10] from the C axis. A Schottky electrode wherein Ni or Ti is deposited on the (0001) Si face is used to fabricate an SBD.
As a technology to solve the above problems in the case Ni or Ti is used as a Schottky electrode material, the present applicant filed for a patent a technique in which Mo and W are used as Schottky electrode materials (Patent document 1). FIG. 2 and paragraphs [0015] to [0035] in the Patent document 1 show the test results of thermal treatment temperature dependence of the SBD and the n factor in the case Mo or W is deposited on the (0001) Si face of 4H-SiC followed by thermal treatment at 300 to 1200° C. As shown in the figure, in the case Mo or W is used as a Schottky electrode material, the SBD can be controlled in the range of 1.0 to 1.3 eV (1.1 to 1.3 eV for Mo, and 1.0 to 1.1 eV for W) while holding the n factor to be 1.05 or less.
SiC has a wider band gap and higher thermal conductivity than those of Si. Consequently, a silicon carbide semiconductor device is also expected for an application requiring an operation in a high-temperature environment. The thermal treatment at the above temperature in the manufacturing process of a device gives a product that can stably operate in a high-temperature environment, has improved properties in a high-temperature environment such as a reduced leak current, and has an improved surge resistance.
Moreover, Patent documents 2 and 3 disclose, based on detailed embodiments, thermal treatment temperature dependence in the case a metal of a Schottky electrode material is deposited on the (0001) Si face followed by thermal treatment. An embodiment shown in FIG. 2 and paragraphs [0021] to [0024] in the Patent document 2 describes that thermal treatment after depositing Ti, Au, and Pd on the (0001) Si face of 6H-SiC gives an alloy layer in an interface between the metals and SiC, and provides improved forward-direction properties by a decrease in the SBH as a result of the thermal treatment.
An embodiment shown in FIG. 1 and paragraphs [0027] to [0041] in the Patent document 3 describes a SBD in which a Schottky electrode is formed on a (0001) Si face of 4H-SiC having a relatively low carrier concentration and simultaneously an ohmic electrode is formed on a (000-1) C face of the silicon carbide having a relatively high carrier concentration by thermal treatment after depositing Mo on the (0001) Si face and the (000-1) C face.
The present inventors examined using Ta as a Schottky electrode material. In practice, the thermal treatment temperature dependence of the SBH and n factor was evaluated for a diode in which Ta was deposited on the (0001) Si face of 4H-SiC and thermal treatment was carried out at 300 to 700° C. As a result, the n factor can be held to be a low value of 1.05 or less as shown in FIG. 4. However, as shown in FIG. 3, the SBH is monotonically reduced to less than 1.0 eV with increasing thermal treatment temperature, and is deviated from the range desired for the reduction of power loss.
In such a background, the present inventors examined a crystal face on which Ta is deposited and contemplated the use of the (000-1) C face as a crystal face.
Patent documents 4 and 5 disclose technologies to form a Schottky electrode on the (000-1) C face. However, in Patent document 4, an Al electrode is used as a Schottky electrode in a detailed embodiment shown in FIG. 1 and paragraphs [0022] to [0035]. Moreover, the thermal treatment temperature dependence of the SBH and the n factor is not mentioned.
In Patent document 5, a Ni electrode or a Ti electrode is used as a Schottky electrode in a detailed embodiment (a column of Embodiment 1). Moreover, the thermal treatment temperature dependence of the SBH and n factor is not mentioned.
Patent document 1: Japanese Patent Application Laid-Open Publication No. 2005-311347
Patent document 2: Japanese Patent Application Laid-Open Publication No. 2000-164528
Patent document 3: Japanese Patent Application Laid-Open Publication No. 2004-221513
Patent document 4: Japanese Patent Application Laid-Open Publication No. 2004-022796
Patent document 5: Japanese Patent Application Laid-Open Publication No. 2005-079339
Non Patent Document 1: IEEE Trans. Electron Devices, March 1993, Vol. 40, third issue, pp. 645-655
An object of the present invention is to provide a method for manufacturing a silicon carbide Schottky barrier semiconductor device provided with a Ta electrode as a Schottky electrode, in which the Schottky barrier height can be controlled to a desired value in a range where power loss is minimized without increasing the n factor.
Another object of the present invention is to provide a silicon carbide Schottky barrier semiconductor device provided with a Ta electrode as a Schottky electrode, in which the n factor is small, the Schottky barrier height is in such a range that power loss is minimized, and the semiconductor device is suitable for an operation in a high-temperature environment.